1. Background—Field of the Invention
The present invention relates to communications systems utilizing reduced complexity receivers. The invention relates to systems where receivers typically have more than one receiving chain, such as in multiple-antenna wireless communications. The invention relates to a method and system for designing and combining transmit signals, in combination with receiving signals using digital signal sub-sampling, and using reduced number of analog receiver chains.
2. Background: Statement of the Problem
Reduction of complexity, cost, size and power consumption of communications systems and communication devices is a set of high priority targets for designers of communications systems. Modern communications receivers, such as multiple-antenna receivers in wireless systems, are traditionally comprised of multiple analog receive chains, which significantly increase receiver's complexity, cost, size and power consumption. It is highly desirable to design receivers with a reduced number of analog receive chains.
Several categories of techniques have been proposed for reduction of the number of analog receive chains below the number of receiver antennas. One category of techniques ignores signals from some of the antennas. Another category of techniques manipulates received signals in analog domain to orthogonalize components of the received signal. Yet another category of techniques uses high-rate digital signal sampling and/or sub-sampling. Most techniques are focused on single-link communications.
Previously proposed techniques suffer from lack of performance in some or all of the previous high priority targets. It is therefore of significant interest to provide a solution to reducing the number of analog receiver chains in receivers with multiple antennas, together with accompanying reduction in receiver complexity, cost, size and power consumption. The performance of the receivers needs to remain as good as of receivers using as many analog receiver chains as the number of receive antennas. The solution needs to be applicable in general multi-user multi-channel communication systems.
3. Background: Prior Art—General
Background material relevant to the present invention has been discussed in the fields treating the following problems: a) Multiple-antenna systems, b) Reduction in number of analog RF/IF chains for wireless receivers, c) Digital signal sub-sampling, d) Transmit signal orthogonality for wireless communications.
Background: Prior Art—Multiple Antenna Systems
Most current wireless communication systems are composed of nodes with transceivers containing a single transmit antenna and a single receive antenna. It was recently shown that the performance, data rate, capacity, coverage, signal-to-noise ratio, frequency reuse and power consumption of individual transceivers/users, as well as of wireless systems with many users, could be significantly improved if individual nodes/transceivers were built with multiple transmit and/or receive antennas. Such transceivers utilize space time signal processing to combat and/or take advantage of the effects of multipath fading and interference of transmitted signals while propagating through multipath-rich wireless channels. Such systems/transceivers are called “smart antenna” transceivers/systems. Smart antenna techniques can significantly improve today's wireless systems, such as cellular and wireless LAN systems using CDMA, TDMA, OFDM or other transmission techniques.
Performance, data rate and capacity improvements with multiple antennas can be accomplished by various processing techniques. Some of the processing techniques are: introduction of diversity gain, diversity combining, beam-forming, interference suppression, space-time coding, and multiple-input multiple-output (MIMO) techniques. Fundamental principles of smart antenna techniques have been described in [“The Impact of Antenna Diversity On the Capacity of Wireless Communication Systems”, by J. H. Winters et al, IEEE Transactions on Communications, vol. 42, No. 2/3/4, pages 1740-1751, February 1994. ]. According to one taxonomy of smart antenna systems, they can be classified into diversity-combining and beam-forming systems. Good overview of antenna processing techniques can be found in [Gesbert, D.; Shafi, M.; Da-shan Shiu; Smith, P. J.; Naguib, A., “From theory to practice: an overview of MIMO space-time coded wireless systems,” Selected Areas in Communications, IEEE Journal on, Volume: 21 Issue: 3, April 2003 Page(s): 281-302. ]. Diversity-combining systems are further classified into time, frequency and space-polarization systems, whereas beam-forming systems are divided into switched and adaptive beam-forming systems. To utilize full potential of smart-antenna systems, it is required that magnitude and phase of signals emanating from individual antennas be preserved before combining them into the resulting optimally received signal. Interference-suppression techniques incorporating multi-antenna receivers with M receive antennas are capable of nulling up to M−1 interferers. MIMO techniques enable N signals to be simultaneously transmitted in the same bandwidth as only one signal, if/when using N transmit antennas, with the transmitted signal then being separated into N respective signals by way of a set of N antennas deployed at the receiver. This was described, for example, in [“Optimum combining for indoor radio systems with multiple users,” by J. H. Winters, IEEE Transactions on Communications, Vol. COM-35, No. 11, November 1987], [“Capacity of Multi-Antenna Array Systems In Indoor Wireless Environment” by C. Chuah et al, Proceedings of Globecom '98 Sydney, Australia, IEEE 1998, pages 1894-1899 November 1998], and [“Fading Correlation and Its Effect on the Capacity of Multi-Element Antenna Systems” by D. Shiu et al, IEEE Transactions on Communications vol. 48, No. 3, pages 502-513 March 2000.].
Multiple-antenna transceivers with smart antenna processing techniques, for example a MIMO system with N transmit and N receive antenna elements, offers N-fold capacity increase relative to single-antenna system. For a fixed overall transmitted power, the capacity offered by MIMOs scales linearly with the number of antenna elements. Specifically, it has been shown that with N transmit and N receive antennas an N-fold increase in the data rate over a single antenna system can be achieved without any increase in the total bandwidth or total transmit power. See, e.g., [“On Limits of Wireless Communications in a Fading Environment When Using Multiple Antennas”, by G. J. Foschini et al, Wireless Personal Communications, Kluwer Academic Publishers, vol. 6, No. 3, pages 311-335, March 1998. ]. In experimental MIMO systems predicated upon N-fold spatial multiplexing, more than N antennas are often deployed at a given transmitter or receiver. This is because each additional antenna adds to the diversity gain and antenna gain and interference suppression applicable to all N spatially-multiplexed signals. See, e.g., [“Simplified processing for high spectral efficiency wireless communication employing multi-element arrays”, by G. J. Foschini, et al, IEEE Journal on Selected Areas in Communications, Volume: 17 Issue: 11, November 1999, pages 1841-1852. ]. Patent application [2005/0175115 A1, Aug. 11, 2005, J, Walton et al., “Spatial Spreading in a Multi-Antenna Communication System”] proposes a method for taking advantage of multipath channels for MIMO systems.
To enable various smart antenna processing techniques, the following is required: a) That both a transmitter and a receiver have multiple antennas, b) That transmit and receive signals/waveforms be separated into a number of derivative sub-signals and processed in special signal processing ways, and c) That derivative sub-signals be distributed to transmit antennas (or from receive antennas), in special ways. Each derivative sub-signal that is transmitted to (or received from) an antenna, has to be identifiable (in magnitude and phase) and separable from other derivative sub-signals that need to be transmitted to (or received from) other antennas.
Since signals obtained from different antennas in smart antenna receivers are required to preserve magnitude and phase, the most straightforward implementation of smart antenna receivers is such that every antenna is followed by its own analog processing RF/IF chain. Each RF/IF chain downcoverts a signal from one antenna to low-IF or to baseband. There, the signal is digitally sampled in time for purposes of baseband digital signal processing. Usually, every RF/IF chain is comprised of amplifiers, one or more filters, one or more mixers/downconverters and an A/D converter (or a pair of A/D converters for complex signals). The existence of more than one analog RF/IF chain increases power consumption, size and cost of transceivers. One RF/IF chain in a single-antenna receiver accounts for about 30% of the receiver cost. This would suggest that a receiver with 4 chains would cost 90% more that a receiver with a single RF/IF chain. For an N-element array, the total number of RF channels required is N. Therefore, the hardware expense and power consumption of such a system is approximately N times those in a single antenna system. Furthermore, arrays with multiple feed lines and complicated RF circuits introduce more circuit noise and thus are more difficult to integrate into a small area. These are significant disadvantages of well known smart antenna transceivers.
It is therefore highly desirable to invent techniques where many receive antennas could share a reduced number of RF/IF chains (or a single chain) without loss of improvements that smart antenna systems offer.
Background: Prior Art—Reduction in Number of RF/IF Chains; Signal Sub-sampling
Several efforts have been made to design receivers with many antennas and with a reduced number of RF/IF chains (or single chain).
One approach [Adachi et al, “A Periodic Switching Diversity Technique for a Digital FM Land Mobile Radio,” IEEE Transaction on Vehicular Technology, November 1978, pp. 211-219.] proposed the use of two antennas at the receiver followed by a switch which enabled the use of a single RF/IF analog processing chain to alternatively process signals coming from the two antennas, and combine them—thus offering the diversity gain. The method is limited in that, at desirable (low) switching rates, it creates digital signal aliasing (spectrum folding effect). Therefore, the switch has to run at undesirably high switching rates. At higher switching rates, large amount of aliased co-channel noise is propagated, significantly reducing operating signal to noise ratio (SNR) of the proposed receiver, thus making it not useful.
The second approach used adaptive loading on the reactive components' passive radiators to each antenna element, to control the individual signal phase before combining [J. Cheng, Y. Kamiya, and T. Ohira, “Adaptive beamforming of ESPAR antenna using sequential perturbation,” in IEEE MTT-S Int. Microwave Symp. Dig., vol. 1, May 2001, pp. 133-136. ]. The drawback of this approach and its derivatives [Dinger, R., “A planar version of a 4.0 GHz reactively steered adaptive array,” IEEE Transactions on Antennas and Propagation, March 1986, pp. 427-431.] is that the signal phase and magnitude information is lost after combining.
In the third approach [S. Ishii, A. Hoshikuki, and R. Kohno, “Space hopping scheme under short range Rician multipath fading environment,” in Proc. IEEE Veh. Technol. Conf., 2000, pp. 99-104. ], the authors proposed a space-hopping scheme to reduce the number of RF/IF chains to one. This system consists of an array antenna and a switch that switches between the antennas repetitively. A major disadvantage of the approach is the existence of multiple delay lines, which replace multiple RF/IF chain, without obvious reduction is complexity, size, cost, and with unclear performance implications.
The fourth approach, called Spatial Multiplexing of Local Elements (SMILE), was presented in [Jonathan D. Fredrick, Yuanxun Wang, and Tatsuo Itoh, “Smart Antennas Based on Spatial Multiplexing of Local Elements (SMILE) for Mutual Coupling Reduction,” IEEE TRANSACTIONS ON ANTENNAS AND PROPAGATION, VOL. 52, NO. 1, JANUARY 2004, pp 106-114. ] and [ Jonathan D. Fredrick, Yuanxun Wang, and Tatsuo Itoh, “A Smart Antenna Receiver Array Using a Single RF Channel and Digital Beamforming,” IEEE, IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 50, NO. 12, DECEMBER 2002, pp. 3053-3058.]. SMILE offers reduction in hardware requirements for the smart antenna system through the use of a single RF/IF chain for all antennas, and of the sub-sampling of a single element of the incoming modulated carrier at a time. Compared to an N-element traditional smart antenna array, the SMILE offers an N-fold reduction in RF hardware, and reduces the power dissipation and circuit size. To avoid aliasing effects (overlapping of modulation spectrum), the minimum switching rate is determined by the Nyquist sampling theory, which is given by Fs=B×N, where B is signal bandwidth, and N is the number of receive antennas. The SMILE approach suffers from the fact that the rate of the switching is substantially high, even though digital sub-sampling is used. In particular, the sampling rate is unacceptably high for multi-channel multi-user systems. For a typical multi-channel cellular system, according to SMILE, the minimally required sampling rate at the antenna switch is Fs=Bsys×N=Bch*Nch*N where N is the number of antennas in the receiver, Bch is single channel bandwidth, and Nch is the number of channels. This causes excessively high power consumption. Using this approach, no further reduction of switching rate is achievable, since it would result in unrecoverable loss of information due to signal aliasing.
The fifth set of approaches is focused on processing signals in analog domain, after the antennas, by providing methods for orthogonalizing signals prior to passing them through the reduced number of analog receiver chains. These approaches are presented in the following patents and patent applications: [2005/0053164 A1, Catreux, Severine et al., Mar. 10, 2005, “System and method for RF signal combining and adaptive bit loading for data rate maximization in multi-antenna communication systems.”]; [2005/0105632 A1, May 19, 2005, Catreux-Erces, Severine et al., “System and method for channel bonding in multiple antenna communication systems.”]; [2006/0029146 A1, Feb. 9, 2006, Catreux; Severine; et al., “Multi-antenna communication systems utilizing RF-based and baseband signal weighting and combining.”]; [U.S. Pat. No. 7,006,810, Winters et al., Feb. 28, 2006, “Method of selecting receive antennas for MIMO systems.”; 2006/0029149 A1, Kim; Hyoun-Kuk et al. , Feb. 9, 2006,” Method and apparatus for receiving signals in MIMO system.”]. This set of approaches incurs significant implementation complexity in analog domain, aggravated by high frequencies at which the methods have to operate. This increases size, cost and power consumption, though potentially reducing the actual number of analog receiver chains.
Background: Prior Art—Signal Orthogonality
Signal orthogonality has been used to facilitate the design of successful wireless communications systems, such as CDMA-based cellular systems and OFDM-based wireless LAN systems. In prior art, signal orthogonality has been utilized to distinguish signals destined to different terminal stations, to distinguish signals transmitted from different base stations, and to reduce interference. The description of techniques and systems using orthogonal signals can be found in: [U.S. Pat. No. 6,553,019 B1, Apr. 22, 2003, Laroia et. al, “Communication System Employing Orthogonal Frequency Division Multiplexing Based Spread Spectrum Multiple Access.”]; [U.S. Pat. No. 6,819,930 Laroia et al., Nov. 16, 2004, “Apparatus and method for use in allocating a channel resource in wireless multiple access communications systems.”]; [U.S. Pat. No. 7,003,021 B2, Feb. 21, 2006, Gillhousen et al., “System and Method for Generating Signal Waveforms in a CDMA Cellular Telephone System.”.], and [U.S. Pat. No. 7,020,110 Walton et al., Mar. 28, 2006, “Resource allocation for MIMO-OFDM communication systems.”]. Although background material on orthogonality is useful for the invention in the present patent, the literature and patents on this topic do not address the issue of complexity reduction in receivers with multiple antennas and multiple analog receive chains.
Bakground: Prior Art—Summary of Disadvantages
Methods described in prior art suffer from one or more of the following inadequacies: a) They run at unaffordably high switching rates—with high power consumption; b) Signal-to-noise ratio is significantly degraded; c) Signal phase and magnitude information is lost; d) Multiple analog chains are replaced by other costly and complex components; e) Received signals experience unrecoverable aliasing; f) None of the known methods addresses or takes advantage of multi-user multi-channel wireless systems' peculiarities.